Exposure to broad band noise can elevate auditory threshold an induce pathological changes in hair cell receptors and nerve terminals in the ear. The degree and reversibility of the changes depend upon the intensity and duration of the exposure. The specific objective of this proposal is to evaluate the ultrastructural changes which occur in hair cells, especially the stereocilia, and in the peripheral endings of auditory nerve fibers during reversible (temporary) elevation of thresholds. Threshold elevations will be assessed by measuring the decrease in amplitude of both the microphonic an neural components of the cochlear potential. Immediately after measuring these decreases the cochleas will be perfused with fixative. The stereocilia will be examined with a light microscope equipped with Nomarski optics and a scanning electron microscope. The internal ultrastructure of the hair cells and nerve endings will be examined with a transmission electron microscope. The ultrastructure of inner ear with reversibly elevated thresholds will be compared with controls to determine the morphological nature and site of intracellular changes. cochleas with reversibly elevated thresholds will also be compared with those showing irreversibly (permanently) elevated thresholds in order to determine whether the differences between them are merely in degree or involve different cellular sites. This proposal fits in with my long-term research objective which is to understand the ultrastructural basis of both normal transduction and diseases affecting the transducer mechanism in the inner ear. Structural changes induced by exposure to loud noise will increase our understanding of normal transduction by determining how specific pathological changes in structure correlate with functional changes. It will also increase our basic understanding of noise induced hearing losses which are clinically very common.